Apparatus and method for remote beam forming for dbs satellites

ABSTRACT

A satellite broadcasting system is achieved where remote beam forming processors combined with wavefront multiplexers located among distributed ground stations are used to control downlink beam footprints and pointing directions. Digital beam forming (DBF) techniques allow a single satellite download broadcast antenna array to generate multiple independently pointed simultaneous downlinks, which may contain distinct information content. Allocation of some uplink back-channel elements as diagnostic signals allows for continuous calibration of uplink channels, improving downlink broadcast array and user broadcast performance. Wavefront multiplexing/demultiplexing allows all array element signals to be radiated by the broadcasting antenna array, with simultaneous propagation from ground stations to the broadcasting satellites through available parallel propagation channels in the uplinks of feeder links, with equalized amplitude and phase differentials. Further, additional wavefront multiplexing/demultiplexing pairs are further used to coherently broadcast signals from a remote beam forming facility on ground to cover areas through multiple broadcasting satellites.

This application is a continuation of application Ser. No. 13/291,594,filed Nov. 8, 2011, now pending, which is continuation-in-part ofapplication Ser. No. 12/122,462, filed on May 16, 2008, now U.S. Pat.No. 8,098,612, which claims the benefit of provisional application No.60/930,943, filed on May 21, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to satellite-based broadcasting systems.More particularly, it relates to systems for downlinking broadcastcontent data from satellites that are linked to ground-based uplinkstations having remote beam-forming capability. Additionally, thepresent invention relates to coherent power combining techniques fordownlinking various broadcast data streams over a common coverage areafrom multiple satellites that are linked to a ground-based uplinkstation.

2. Description of Related Art

Systems for broadcasting audio and video data from satellites ingeostationary orbit (GEO), medium-Earth orbit (MEO), and low-Earth orbit(LEO) to ground-based users are well known in the art. Such systemsgenerally rely on one or more ground-based uplink facilities to uplinkcontent to the satellite. The satellite then downlinks the content toindividual users in selected geographical areas. Some systems employshaped reflectors to develop a downlink beam pattern to illuminate theselected region. Others employ multiple antennas or antennas withmultiple elements in order to configure the footprint of the downlinkbeams.

However, nearly all of such systems are fixed at the time the satelliteis launched and cannot be upgraded to keep pace with more advancedground systems. Even those systems that maintain some satelliteconfigurability must be controlled by complex systems on board thesatellite and thus have inherently limited flexibility. Thus, it wouldbe useful to provide a system with a radiation pattern that can beconfigured and controlled by ground-based uplink stations. Inparticular, it would be useful to provide a system that uses remoteground-based digital beam forming (DBF) systems to causedirect-broadcast satellites to radiate multiple downlink beams withconfigurable shapes pointing in configurable directions.

SUMMARY OF THE INVENTION

A satellite broadcasting system is achieved that provides multiplesimultaneous downlinks controlled by remote digital beam forming (DBF)processors employed at distributed, ground-based uplink stations. Thesystem enables the formation of configurable downlink radiation patternscovering selected service areas, enables the use of multiplesimultaneous downlinks that may carry different content to differentgeographic regions, and minimizes the impact of differential phase andamplitude drift between uplink signal elements.

An embodiment of a satellite broadcasting system in accordance with thepresent invention includes a satellite segment, a ground segment, and auser segment. The satellite segment includes one or more satelliteshaving a downlink broadcast antenna array. An embodiment of such adownlink broadcast antenna array having ten radiating elements isdescribed in detail below, but more generally, the downlink broadcastantenna array comprises N elements, where N is a positive integergreater than one.

The satellite segment also includes a wavefront de-multiplexer having atleast N outputs configured to drive corresponding ones of the N downlinkbroadcast antenna array elements. The wavefront de-multiplexer is adevice with M inputs and M outputs, where M is an integer greater thanor equal to N and configured to perform a spatial Fourier transform ofits inputs. N of the M transformed outputs are used to drive the Ndownlink broadcast antenna array elements. If M is greater than N, theremaining M−N outputs of the wavefront de-multiplexer are used to drivean optional cost-function unit that is adapted to measure imbalances inuplink back-channels used to uplink audio, video or other content to thesatellite segment for subsequent downlink to the user segment. Theoperation of the optional cost function unit is described in more detailbelow.

The wavefront de-multiplexer can be implemented in a number of ways,including by employing an M-by-M Butler Matrix, well known in the art.In the detailed description that follows, a system is described thatuses a 16-by-16 Butler Matrix, but more generally, an M-by-M ButlerMatrix may be used where M is an integer greater than or equal to N. Theinputs to the M-dimensional wavefront de-multiplexer are produced by afrequency-domain de-multiplexer that operates on frequency-domainmultiplexed (FDM) uplink signals received from the ground segment. TheFDM signals from the ground segment are de-multiplexed into Mcomponents, and each of the M components is frequency converted to aselected downlink frequency for subsequent transmission over thedownlink broadcast antenna array.

The ground segment comprises one or more ground terminals for uplinkingaudio, video, or other content to the satellite segment for subsequentdownlink to the user segment. An embodiment of a ground terminal inaccordance with the present invention includes at least one digital beamforming (DBF) processor that is configured to encode amplitude and phaseinformation onto a data stream such that a coherent beam is formed bythe downlink broadcast antenna of the satellite segment. A stream ofbaseband content data is multiplied by an N-component beam weight vector(BWV) to create an N-component product vector. An embodiment describedin detail below uses a ten-component BWV corresponding to the tenelements of the downlink broadcast antenna array in that embodiment.However, more generally, a system including an N-element downlinkbroadcasting antenna array on a broadcast satellite will use N-componentBWVs to properly weight baseband content data.

The N-component product vector is then padded with nulls to create anM-component product vector, where M is an integer greater than or equalto N. For example, in an embodiment described in detail below, M issixteen and N is ten. The M-component product vector, consisting of Nsignals and M−N nulls, is then processed by an M-by-M wavefrontmultiplexer that performs an M-component spatial Fourier transform. TheM-component output of the wavefront multiplexer is then passed through Manalog-to-digital converters to produce M analog waveforms. Each of theM analog waveforms is frequency up-converted to a different frequencynear the selected uplink frequency. In an embodiment described in detailbelow, the uplink center frequency is selected to be 6 GHz, and thesixteen analog waveforms are up-converted to frequencies spaced by 62.5MHz and extending from 5.5 GHz to 6.5 GHz. More generally, however, thecenter frequency can be selected to be any frequency known to be usefulfor satellite communications, such as S-band, C-band, X-band, Ku-band,or Ka-band. The spacing between the frequencies likewise may be selectedaccording to the bandwidth requirements of the application.

The M up-converted signals are then combined into frequency-domainmultiplexed (FDM) uplink signals, and these uplink signals aretransmitted as back-channel signals to the satellite segment through afeeder link at C-band, in this embodiment. After processing through thesatellite segment as described previously, the signals encoded with theBWV are applied to the elements of the downlink broadcast antenna array.The amplitude and phase profiles encoded in the BWV create a beam thatadds coherently in a particular direction and that exhibits a particularpattern shape. Changing the BWV coefficients applied by the groundsegment thus changes the pointing of the downlink from the satellitesegment. Within the ground segment, multiple content streams may bemultiplied by multiple BWVs to create multiple beams when the signalsare subsequently applied to the satellite segment downlink broadcastantenna array. Thus, the ground segment controls the pointing andshaping of multiple simultaneous beams downlinked from the satellitesegment.

For the case in which M is chosen to be larger than N such that theN-component product vector created in the ground segment is padded byone or more nulls, the cost function unit mentioned above allowscalibration and monitoring of the uplink channel, enabling improveddownlink performance of the system. When the back-channel signalsembedded in the uplinked beam is received and amplified by the satellitesystem, frequency converted to the broadcast frequency (typically Ku orS band), and run through the wavefront de-multiplexer, the original Ndata streams (each modulated by corresponding components of the BWV) arerecovered, including the M−N null channels. For a completely balancedfeeder link with multiple back channels having identical propagationdelays and attenuations, the M−N recovered channels will contain nosignals. However, in reality, imbalances and differential propagationcharacteristics among various channels will cause some energy to leakinto the null outputs of the wavefront de-multiplexer. By monitoring thenull channels, changes can be made dynamically to the amplitudes andphases of the signals entering the wavefront de-multiplexer to correctfor these imbalances and to produce true nulls where expected.Alternatively, the null energy monitored by the cost function unit canbe downlinked back to the ground segment via a separate backchannel, andthe ground segment can accordingly pre-compensate for the measuredimbalances by adjusting the amplitudes and phases of the signalsemerging from the wavefront multiplexer on the ground segment. Thus, theuse of one or more null channels in the uplink enables continuouscalibration and monitoring of the uplink channel, assuring that thedownlink beams are formed properly and cleanly.

By contrast, without the wavefront multiplexing feature provided by thepresent invention, each weighted element signal would propagate througha unique backchannel in the feeder link. The differential propagationcharacteristics of each channel would modulate the weighted elementsignals differently in both amplitude and phase. Depending on thefrequency band of the feeder link, these effects could significantlydistort the shape of the broadcast beam.

The wavefront multiplexing scheme, however, directs each of the Nweighted element signals generated by the remote beam-forming facilityon the ground to go through all of the M propagation channelssimultaneously and in parallel. Thus, the channel effects are spreadacross the entire feeder link bandwidth and across all of the N weightedelement signals, reducing the potential distortion effects by a factorof the square root of M, or four for the example discussed below havingM equal to sixteen.

From the foregoing discussion, it is clear that certain advantages havebeen achieved for a satellite broadcast system that utilizesground-based remote digital beam forming. Further advantages andapplications of the invention will become clear to those skilled in theart by examination of the following detailed description of thepreferred embodiment. Reference will be made to the attached sheets ofdrawing that will first be described briefly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a direct broadcast system comprising a ground segment, asatellite segment, and a user segment in accordance with the presentinvention;

FIG. 2 is a block diagram of an embodiment of the ground segment of asatellite broadcast system in accordance with the present invention;

FIG. 3 is a block diagram of an embodiment of the satellite segment of asatellite broadcast system in accordance with the present invention; and

FIG. 4 depicts an example of a satellite broadcast system in whichmultiple distributed ground stations communicate with a satellitesegment to produce multiple simultaneous downlinks in accordance withthe present invention.

FIG. 5 depicts an example of back channel up-link calibrations for thefeeder link implemented via pre-compensations on ground for a satellitebroadcast system in accordance with the present invention.

FIG. 5A depicts a portion of a back-channel down-link interface of thefeeder-link of FIG. 5 in which an example of the back channel up-linkcalibrations for the feeder-link are implemented via pre-compensationson ground of a satellite broadcast system in which in accordance withthe present invention.

FIG. 5B depicts three calibration functions boxes of FIG. 5 in which anexample of feeder link calibrations for the back channel up-link areimplemented via pre-compensations on ground of a satellite broadcastsystem in accordance with the present invention; (a) a satellite bornetransmission functions via a separated WF muxing for the down linkinterface of the feeder-link, (b) a ground based receiving WF demuxingfunctions compensating for down link propagation differentials of thefeeder-link, and (c) a ground based pre-compensation functions for uplink propagation differentials of the feeder-link.

FIG. 5C depicts the calibration functions of FIG. 5 in which an exampleof feeder link calibrations for the back channel up-link are implementedvia pre-compensations on ground of a satellite broadcast system inaccordance with the present invention; assuming (1) the onboard WFdemuxed signals are available on the ground processing facility, (2) thepropagation differentials after the WF demuxer in FIG.5 are properlycompensated by modifications on the BWVs of the DBFs.

FIG. 6 depicts an example of feeder link calibrations for the backchannel up-link implemented via satellite on-board compensations for asatellite broadcast system in accordance with the present invention.

FIG. 7 depicts an example of feeder link calibrations for theback-channel up-link implemented via satellite on-board compensationsfor a satellite broadcast system in accordance with the presentinvention.

FIG. 8 depicts an example of coherent power combining of twobroadcasting satellites via WF muxing in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention provides a direct satellite broadcast system that includesa remote ground-based beam-forming facility. Beam-forming information isencoded into audio, video, or other content within the ground-baseduplink facility. It is then uplinked to one or more satellites anddownlinked via a segmented downlink broadcast antenna array.Beam-forming information encoded on the ground creates multiple downlinkbeams appearing at the satellite's downlink broadcast antenna array,allowing control over which content is radiated to which ground-basedusers.

FIG. 1 depicts a block diagram of a direct broadcast system inaccordance with the present invention. The ground segment 116 includes afirst digital beam forming (DBF) processor 124 and a second DBFprocessor 126. Content data 140 enters the first DBF processor 124, andcontent data 142 enters the second DBF processor 126. The two contentstreams may be identical or may be distinct, depending on the particularbroadcast application. The first DBF processor applies a set of beamweight vectors (BVWs) to the content data 140 that correspond to aparticular selected footprint for a first beam 130 that will bedownlinked from the satellite segment 102. The second DBF processor 126similarly applies an independent set of BWVs to the content data 142that correspond to a separate distinct footprint for a second beam 132that will be downlinked by the satellite segment 102. The content data,now encoded with beam footprint and pointing information, is sent to awavefront multiplexer 122. The wavefront multiplexer 122, described morefully with respect to FIG. 2 below, performs a spatial Fourier transform(FT) of the encoded content data to produce multiple baseband uplinksignals. The multiple baseband signals are then frequency up-converted120 to several closely-spaced uplink carriers and combined into acomposite frequency-domain multiplexed (FDM) uplink signal. To savebandwidth, two FDM multiplexers may be used to create two half-bandwidthstreams, each with an orthogonal polarization such as vertical andhorizontal linear polarization, or right-hand circular polarization(RHCP) and left-hand circular polarization (LHCP). The uplink signal isthen amplified 118 and radiated to one or more satellites via an uplinkantenna 128. It should be noted that while the above description refersto a ground segment including two DBF processors for encoding twocontent data streams, other numbers of DBF processors are possible andwould fall within the scope and spirit of the present invention.

The satellite segment 102 receives the uplinked FDM signal via an uplinkreceive antenna 114. The uplink signal is amplified 112 and sent throughan FDM receiver 110 that de-multiplexes the closely spaced carriers andfrequency converts them to the satellite segment downlink frequency. Thede-multiplexed and frequency-converted signals are then passed through awavefront de-multiplexer 108, described more fully with respect to FIG.3 below, that performs a spatial FT of the received signals. The outputsof the wavefront de-multiplexer 108 are amplified 106 and routed toelements of a downlink broadcast antenna array 104. The BWVs that wereencoded into the two beams by the ground segment facility 116 operate togenerate two independent downlink beams 130 and 132 in the user segment134. The footprints of the two downlink beams 130 and 132 can becontrolled by the ground segment 116 to illuminate geographicallyseparated users 136 and 138. The ground segment 116 can also controlwhich digital content is sent to user groups 136 and 138 by varying theinputs 140 and 142 to the two DBF processors 124 and 126.

It should be noted that while the embodiment described above employedtwo DBF processors to encode two uplink content streams, more than twoDBF processors and content streams may also be used to create more thantwo downlink beams. Such systems employing multiple DBF processors wouldalso fall within the scope and spirit of the present invention.

FIG. 2 presents a more detailed block diagram of an embodiment of aground segment portion of a digital broadcast system in accordance withthe present invention. The two content data streams 140 and 142 aredepicted as inputs to a first DBF processor 202 and a second DBFprocessor 206. In the embodiment depicted in FIG. 2, the digital contentstream 140 is split into ten portions that are multiplied by componentsof a ten-dimensional first BWV 204. Each of the ten components of thefirst BWV 204 is a complex number containing both an amplitude piece anda phase piece. The specific values of the BWV are chosen to create aparticular beam footprint from the downlink transmission antenna of thesatellite segment, as will be more fully described below with referenceto FIG. 3. The content data stream 140 is multiplied 214 by the firstset of BWVs 204, and the encoded contents are then routed to a wavefrontmultiplexer 216.

At the same time, the second input content stream 142 is multiplied by asecond set of BWVs 208 in the second DBF processor 206 to create anindependent second beam in the satellite segment. Note that altering thevalues of the second set of BWVs 208 will change the footprint andpointing direction of the second beam generated by the satellite segmentbut will not have an effect on the first beam. After multiplication bythe second BWV 208, the outputs of the second DBF 206 are combined withthose of the first DBF 202 on an element-by-element basis, and thecombination is routed to the inputs of the wavefront multiplexer 216.

The wavefront multiplexer 216 takes the input elements, encoded withamplitude and phase pointing data, and performs a spatial Fouriertransform (FT). The wavefront multiplexer 216 can be implemented in manyways, such as by a sixteen-by-sixteen Butler Matrix, well known in theart. Note that for the embodiment depicted in FIG. 2, only ten of theinputs of the wavefront multiplexer 216 are used while all sixteenoutputs are used. The remaining six inputs are set to null inputs, asshown schematically at 218. As will be described in more detail belowwith reference to FIG. 3, the zero-input channels will allow forcontinuous calibration, equalization, and optimization of the dynamicpropagation effects of the uplink back-channels in the feeder link,enabling improved system performance Wavefront multiplexers of sizesother than sixteen by sixteen may also be used, different numbers ofunused inputs may also be employed, and continuous calibration may beperformed as long as the number of wavefront multiplexer signal inputsis less than the number of outputs. In more general terms, the number ofsignal inputs to the wavefront multiplexer will be N, and the number ofoutputs will be M, where N and M are positive integers greater than one.When M is greater than N, continuous calibration techniques may be used,as the number of null inputs to the wavefront multiplexer will be M−N.The downlink broadcasting antenna array 104 will comprise N elements,corresponding to the N signal inputs to the wavefront multiplexer. Suchalternately sized systems would also fall within the scope and spirit ofthe present invention.

By forming a Fourier transform, the wavefront multiplexer has the effectof mixing each of the ten inputs into each of the sixteen outputs. Thus,each of the ten input element signals goes through all of the sixteenoutput channels, and each of the zero-input channels also goes throughall of the sixteen output channels. In the subsequent ground processing,uplinking to the satellite element, and processing in the satellite,portions of the null-input signals thus sample each of the sixteenuplink channels and can be used to correct for dynamic channel-specificpropagation effects and imbalances.

The outputs of the wavefront multiplexer 216 are run throughdigital-to-analog converters 216 that synthesize analog waveforms. Theanalog waveforms are then frequency up-converted 224 by a series ofclosely spaced carrier frequencies in the feeder-link uplink frequencyband and generated by frequency sources 220. The uplink frequency may beany frequency spectrum useful for satellite communications, such asS-band, C-band, X-band, Ku-band, or Ka-band. For purposes ofillustration, the uplink will be assumed to be C-band at 6 GHz. Thesixteen output channels are then frequency up-converted by sixteencarriers separated by 62.5 MHz and extending from 5.75 GHz to 6.25 GHz.Other channel spacings can be used depending on the bandwidthrequirements of the application. The separate carriers are then combinedinto two 500-MHz-wide C-band signals by low-loss frequency-domainmultiplexers (FDMs) 226. The FDM carrier is then amplified 228 by asolid-state power amplifier, traveling wave-tube amplifier (TWTA),klystron, or other radio-frequency amplifier known in the art, andtransmitted via an uplink transmit antenna 128. The two 500-MHz C-bandsignals are uplinked to the designated satellite using polarizationdiversity. For example, one signal may be horizontally polarized and theother vertically polarized. Or one may be right-hand circularlypolarized (RHCP) and the other left-hand circularly poralized (LHCP).

Note that the ground segment described above may operate by itself or inconjunction with multiple ground segments having similar configurations.As long as their uplink frequencies are separated, multiple groundsegments can be used to uplink content to the same satellite or group ofsatellites.

FIG. 3 depicts a more detailed view of an embodiment of the satellitesegment of a direct broadcast system in accordance with the presentinvention. The uplink receive antenna 114 receives the C-band FDM uplinksignal from the ground segment depicted in FIG. 2. Note that frequenciesother than C-band may be used for the uplink feeder links and still fallwithin the scope and spirit of the present invention. On thebroadcasting satellite, the two FDM uplinked signals are amplified bylow-noise amplifiers 302 and sent to two FDM de-multiplexers 304 thatsplit the 6 GHz FDM signals into constituent carriers, spaced 62.5 MHzapart. Of course, different frequency spacings of the constituent piecesof the uplink carrier may be used within the scope of the presentinvention.

The sixteen individual components of the uplink carrier are then passedthrough sixteen frequency converters 306 that frequency-shift the uplinkelements to the broadcast downlink frequency, here assumed to be S-band.However, other satellite broadcast downlink frequencies such as Ku-bandor Ka-band may also be used.

The outputs of the frequency converters 306 are the sixteen channelsignals, all at S-band in this embodiment. The sixteen channels arepassed through band-pass filters 308 and then enter a wavefrontde-multiplexer 312 that performs a spatial Fourier transform (FT) of thesixteen uplinked channels. The transformation performed in the wavefrontde-multiplexer 312 is essentially the opposite of that performed in thewavefront multiplexer 216 in the ground segment. Thus, ten elementsignals are recovered, are filtered by band-pass filters 318, areamplified by solid-state power amplifiers 320 or other radio-frequencyamplifiers, and are routed to corresponding radiating elements of thedownlink broadcast antenna array 322. The amplitude and phase profilesimparted to the signals by the ground-based DBF processors then causethe fields radiated by the downlink broadcast antenna array 322 tocombine constructively and destructively coverage areas. The satellitesegment itself is relatively uncomplicated and simply passes through thebeam-forming encoding generated by the ground segment. This keeps mostof the complexity and control on the ground, where it is easilyaccessible for upgrades, maintenance, and reconfiguration as required.

As the sixteen uplink element signals pass through the wavefrontde-multiplexer 312, the six null channels 314 are also recovered alongwith the ten signal channels. If all of the processing and propagationchannels for the sixteen radiated signal elements were identical, thenull channels would output nothing. However, imbalances in attenuationlevels, phase delays and other propagation effects of the various uplinkback channels will tend to cause energy to leak into the null channels314. Thus, they become observables that can be used to measure andcorrect for imbalances in the uplink back channels. The null channels314 are routed to a cost function processor 316 that calculatescompensation weight vectors (CWVs) to compensate for propagationeffects. These correction vectors may be applied on board the satellitesegment to correct the received uplink signals, or alternatively, couldbe downlinked back to the ground segment via a separate backchannel toallow the ground segment to pre-compensate the uplink elements beforetransmission to the satellite.

As noted previously, a system in accordance with the present inventionneed not employ ten signal channels, sixteen uplink element signals, andsix null channels. In more general terms, a system may comprise N signalchannels and M uplink element signals, where M and N are positiveintegers greater than one. If the continuous calibration method is to beused, M must be greater than N, and the number of null channels is equalto M−N. However, systems for which M is equal to N and no continuouscalibration is performed would also fall within the scope and spirit ofthe present invention.

FIG. 3 depicts a single uplink-downlink processor on board the satellitedriving the downlink broadcast antenna array 322. However, multipleuplink-downlink processors may be used simultaneously with the samedownlink broadcast antenna array 322. FIG. 4 illustrates an embodimentof such a system including multiple satellite payloads all driving thesame downlink broadcast antenna array 322. The embodiment depicted inFIG. 4 includes a satellite segment 402 that comprises a downlinkbroadcast antenna array 322 driven by a set of power amplifiers 414 thatare themselves driven by signals from a first 404, second 406, and third408 payload element. Each of the payload elements 404, 406, and 408,includes the components depicted in the system of FIG. 3.

As depicted in FIG. 4, the satellite segment 402 interacts with a groundsegment that includes three ground stations 424, 426, and 428. Each ofthe ground stations generates an uplink signal 430, 432, and 434, thatis distinct in broadcasting frequency from the uplink signals generatedby the other two ground stations. In the embodiment depicted here, twoof the uplinks are directed toward an uplink receive antenna 410 that isshared by the first payload element 404 and the second payload element406. The uplink frequency difference between uplinks 430 and 432 allowthe satellite segment to distinguish between the two. The first uplink404 is processed by the first payload element 404 to generate ten drivesignals that are sent to the downlink broadcast antenna array 322.Similarly, the second uplink 432 is processed by the second payload 406to generate an additional ten drive signals that are also applied to thedownlink broadcast antenna array 322. Finally, the third uplink 434, isreceived by a second satellite-segment uplink-receive antenna 412, whichmight be pointing to a different coverage area than the otheruplink-receive antenna 410. The third uplink 434 is processed by thethird payload element 408 and an additional ten drive signals areproduced that are amplified by the amplification stage 414 and routed tothe same downlink broadcasting antenna array 322. The three beams can becontrolled independently and may broadcast different content data eitherby maintaining spatial separation of coverage areas while using the samefrequency spectral band, overlapped coverage areas withfrequency-separated broadcasting channels, or a combination of both.When overlapping coverage areas are used, other techniques, such asorthogonal coding, e.g. CDM, or time domain multiplexing (TDM) can beused.

The digital beam formers in each of the ground stations 424, 426, and428 encode their respective uplinks with proper phase and amplitudeprofiles such that when converted to downlink frequency and radiated bythe downlink broadcast array 322, the signals from downlink elementswill add nearly coherently only in the selected coverage area for eachof the beams created by the ground stations 424, 426, and 428. Thus, thedownlink broadcast antenna array is able to radiate multiple,simultaneous beams 416, 418, 420, and 422 pointing to independentcoverage areas, including different broadcast data, and optionallyincluding broadcasting beams with slowly configurable coverage areas toaccommodate satellites in slightly inclined geostationary orbits. Thus,the complexity associated with pointing multiple simultaneous beams,establishing appropriate footprints, and managing which content isdownlinked to which geographic regions is largely controlled by theground segment. This simplifies the satellite segment and greatlyimproves the configurability of the direct broadcast system.

Thus, a direct broadcast system is achieved in which remote beam formingprocessors located among distributed ground stations may be used tocontrol downlink beam footprints and pointing directions. Digital beamforming techniques allow a single satellite downlink broadcast antennaarray to generate multiple simultaneous downlink beams that can beshaped and pointed independently and that may carry distinct informationcontent. By allocating some of the uplink channel elements as nullchannels, continuous calibration of the uplink channel can be performed,improving the performance and quality of the downlink broadcastingefficiency for user segments. Those skilled in the art will likelyrecognize further advantages of the present invention, and it should beappreciated that various modifications, adaptations, and alternativeembodiments thereof may be made within the scope and spirit of thepresent invention. The invention is further defined by the followingclaims.

FIG. 5 depicts a block diagram of a direct broadcast system with 4functional expansions of FIG. 1 in accordance with the presentinvention. The 4 functional expansions are implemented in both groundand satellite segments

-   -   (1) A programmable equalizer 522A is inserted in the ground        segment between the functional box of WF mux 122 and that of FDM        Mux/Freq. Conversion 120;        -   a. It is serviced as a pre-compensating processor altering            amplitudes and phases of signals passing through        -   b. via a weighting process by a compensation weighting            vector (CWV)    -   (2) a receiving function interface, G2, is implemented in the        ground segment as a part of the back-channel downlink functions        of the feeder-link via the ground antenna 128;    -   (3) channelized received data at the point ‘A’ in between the        functional block of the FDM Demux/Freq. Conversion 110 and that        of WF demux 108 the on satellite is referred to as the onboard        prime data, which will be transported to downlink (D/L) back        channels of the feeder-link, and    -   (4) a transmitting function interface, A2, is implemented in the        satellite segment as a part of the D/L back channels of the        feeder-link via the satellite antenna 114.

There are two pairs of WF mux/demux processing; one inside the other.The outer one is for calibration of backchannel for U/L data transferconsisting of the WF mux 122 on ground and WF demux 108 on satellite.Programmable equalizer 522A is placed on ground as a pre-distortionmechanism compensating for anticipated up-link propagationdifferentials. The inner pair features a downlink (D/L)WF mux 513 onsatellite and a D/L WF demux 533 on ground with a programmable equalizer533A as a post-compensation mechanism for the D/L back channelpropagation differentials. There is an additional WF demux processing543 and associated programmable equalizer 543A on ground emulatingcompensating effects of the equalizer 522A in the main path for the backchannels in uplink. They are part of an optimization loop 540 for theuplink equalization. The resulting CWV for the equalizer 543A will becopied to the pre-distortion equalizer 522A on a frame to frame basis.

Ground segment 116 includes a first digital beam forming (DBF) processor124 and a second DBF processor 126. Content data 140 enters the firstDBF processor 124, and content data 142 enters the second DBF processor126. The two content streams may be identical or may be distinct,depending on the particular broadcast application. The first DBFprocessor applies a set of beam weight vectors (BVWs) to the contentdata 140 that correspond to a particular selected footprint for a firstbeam 130 that will be downlinked from the satellite segment 102. Thesecond DBF processor 126 similarly applies an independent set of BWVs tothe content data 142 that correspond to a separate distinct footprintfor a second beam 132 that will be downlinked by the satellite segment102. The content data, now encoded with beam footprint and pointinginformation, is sent to a wavefront multiplexer 122. The wavefrontmultiplexer 122, performs a spatial Fourier transform (FT) of theencoded content data to produce multiple baseband uplink signals. Theoutputs of the spatial FT are modified by compensation weighting vectors(CWV) 522A as a pre-distortion means to equalize propagationdifferentials to be encountered when propagating through feeder linksbetween a ground antenna 128 and a satellite antenna 114. CWV weighting522A is usually implemented digitally by a bank of digital complexmultipliers and are controlled by an optimization processor 540. Thebank of multipliers is usually a part of a digital processor performingthe DBFs and WF muxing functions.

The multiple baseband signals are then frequency up-converted 120 toseveral closely-spaced uplink carriers and combined into a compositefrequency-domain multiplexed (FDM) uplink signal. To save bandwidth, twoFDM multiplexers may be used to create two half-bandwidth streams, eachwith an orthogonal polarization such as vertical and horizontal linearpolarization, or right-hand circular polarization (RHCP) and left-handcircular polarization (LHCP). The uplink signal is then amplified 118and uplinked to one or more satellites via the antenna 128. It should benoted that while the above description refers to a ground segmentincluding two DBF processors for encoding two content data streams,other numbers of DBF processors are possible and would fall within thescope and spirit of the present invention. Generally speaking a DBFprocessor may generate many concurrent beams. We shall refer a Tx DBFprocessor as a P-to-N Tx DBF processor which features P inputs and Noutputs. The P input corresponds to P input-beams, and N outputs for theN antenna elements.

The satellite segment 102 receives the uplinked FDM signal via an uplinkreceive antenna 114 for a feeder-link. The uplink signal is amplified112 and sent through an FDM receiver 110 that de-multiplexes the closelyspaced carriers and frequency converts them to the satellite segmentdownlink frequency. The de-multiplexed and frequency-converted signals,identified by “A” as “pre-WF demuxer signals,” are divided into twopaths. The pre-WF demuxer signals featuring M individual streams via thefirst path 150 are sent to an onboard N′-to-N′ WF muxer 513, where N′>Mand N′ inputs also consisting of N′−M pilots inputs initiated onsatellite. The WF muxer 513 is a part of the on-board down-link unit 510for the feeder-link. The N′ outputs of the WF muxer 513 arefrequency-converted and FDM muxed by an FDM muxer 514, amplified by adownlink transmitter 515, and then radiated by the feeder-link antenna114.

The on board prime data in the second path from the FDM receiver 110 arepassed through a wavefront de-multiplexer 108, that performs a spatialFT of the received signals. The outputs of the wavefront de-multiplexer108 are amplified 106 and routed to elements of an downlink broadcastantenna array 104. The BWVs that were encoded into the two beams by theground segment facility 116 operate to generate two independent downlinkbeams 130 and 132 in the user segment 134. The footprints of the twodownlink beams 130 and 132 can be controlled by the ground segment 116to illuminate geographically separated users 136 and 138. The groundsegment 116 can also control which digital content is sent to usergroups 136 and 138 by varying the inputs 140 and 142 to the two DBFprocessors 124 and 126.

The downlink back-channels of the feeder-link on ground featurescaptured signals by the ground antenna 128, and sent to a downlink (D/L)processor 530, in which the received signals are amplified by a D/Lreceiver 531, demuxed and frequency converted by a FDM demuxer 532 intoN′ signal streams, which are subsequently frequency converted to acommon carrier, and then connected to a programmable equalizer 533A forcompensating the path differential by a D/L CWV (down link compensationweight vector). There is an optimization loop, converting outputsassociated with satellite initiated pilot signals from the D/L WFDemuxer 533 to a cost function generator 535 to map the currentperformance status into positively defined performance indexes, or costfunctions. As part of an optimization processor 534, the cost functionsare used to dynamically form “Total Cost” for D/L, and measurement ofcost gradients. The optimization processor 534, based on a costminimization algorithm, calculates updated D/L CWV continuously. Atsteady state, total D/L cost shall become zero or negligibly small. Asresult, the updated D/L CWVs shall remain the same, and the M-streams ofonboard pre-WF demuxer signals 550 are recovered on ground.

Based on the recovered onboard prime data 550, there is anotheroptimization loop to calculate current coefficients of thepre-distortion equalizer. The optimization loop on ground features:

-   -   1. a WF demuxer 543 to emulate performance of on board WF        demuxer 108, converting outputs associated with ground initiated        diagnostic signals from the WF muxer 122    -   2. a cost function generator 545 mapping the recovered        diagnostic signals from the WF muxer 122 into cost functions as        a set of performance indexes,    -   3. an optimization processor 544 formulates a total cost,        performs measurements of the cost gradients, calculated new U/L        CWV, and then send the new CWV for updating in the programmable        equalizer 543A. At steady state, total U/L cost shall become        zero or negligibly small. As result, the updated U/L CWVs shall        remain the same.    -   4. the optimization loop 540 also send the current new CWV to        the equalizer 522A in the main uplink paths on a frame-by-frame        basis.

It should be noted that while the embodiment described above employedtwo DBF processors to encode two uplink content streams, more than twoDBF processors and content streams may also be used to create more thantwo downlink beams. Such systems employing multiple DBF processors wouldalso fall within the scope and spirit of the present invention. Suchsystems employing multiple DBF processors would also fall within thescope and spirit of the present invention. Similarly, one DBF processorand multiple content streams may also be used to create multiplebroadcasting downlink beams; each associated with different contentstreams and covering various service areas. Such systems employing asingle DBF processor with multiple coverage areas for various contentstreams would also fall within the scope and spirit of the presentinvention.

FIG. 5A depicts a block diagram of a direct broadcast system with 4functional expansions of FIG. 1 in accordance with the presentinvention. The 4 functional expansions are implemented in both groundand satellite segments

-   -   (1) a programmable equalizer 522A inserted in the ground segment        between the functional box of WF mux 122 and that of FDM        Mux/Freq. Conversion 120;    -   (2) a receiving function interface, G2, implemented in the        ground segment as a part of the downlink (D/L) back channels of        the feeder-link via the ground antenna 128;    -   (3) channelized received data on the first satellite at the        point ‘A’ in between the functional block of the FDM Demux/Freq.        Conversion 110 and that of WF demux 108, referred to as the on        board primedata,    -   (4) a transmitting function interface, A2, is implemented in the        satellite segment as a part of the D/L back channels of the        feeder-link via the satellite antenna 114.

Depicted is only one of the two sets of WF mux/demux processing. Theouter one consists of the WF mux 122 on ground and WF demux 108 on thefirst satellite featuring the programmable equalizer 522Apre-compensating anticipated up-link propagation differentials. Theinner one (not shown) features a D/L WF mux on satellite interfacingwith “A2” and a D/LWF demux on ground interfacing with “G2.” There is anadditional WF demux on ground interfacing with “G1.”

The ground segment 116 includes a first digital beam forming (DBF)processor 124 and a second DBF processor 126. Content data 140 entersthe first DBF processor 124, and content data 142 enters the second DBFprocessor 126. The two content streams may be identical or may bedistinct, depending on the particular broadcast application. The firstDBF processor applies a set of beam weight vectors (BVWs) to the contentdata 140 that correspond to a particular selected footprint for a firstbeam 130 that will be downlinked from the satellite segment 102. Thesecond DBF processor 126 similarly applies an independent set of BWVs tothe content data 142 that correspond to a separate distinct footprintfor a second beam 132 that will be downlinked by the satellite segment102. The content data, now encoded with beam footprint and pointinginformation, is sent to a wavefront multiplexer 122. The wavefrontmultiplexer 122 performs a spatial Fourier transform (FT) of the encodedcontent data to produce multiple baseband uplink signals. The outputs ofthe spatial FT are modified by a U/L programmable equalizer 522A via itscompensation weighting vector (CWV). The programmable equalizer 522Aserves as a dynamic pre-distortion means to equalize propagationdifferentials to be encountered when propagating through uplink backchannels of the feeder links between a ground antenna 128 and asatellite antenna 114. The programmable equalizer 522A featuring CWVweighting is usually implemented digitally by a bank of digital complexmultipliers and is controlled by an optimization processor 540. The bankof multipliers is usually a part of a digital processor performing theDBFs and WF muxing functions.

The multiple baseband signals are then frequency up-converted 120 toseveral closely-spaced uplink carriers and combined into a compositefrequency-domain multiplexed (FDM) uplink signal. To save bandwidth, twoFDM multiplexers may be used to create two half-bandwidth streams, eachwith an orthogonal polarization such as vertical and horizontal linearpolarization, or right-hand circular polarization (RHCP) and left-handcircular polarization (LHCP). The uplink signal is then amplified 118and uplinked to one or more satellites via the antenna 128. It should benoted that while the above description refers to a ground segmentincluding two DBF processors for encoding two content data streams,other numbers of DBF processors are possible and would fall within thescope and spirit of the present invention.

The satellite segment 102 receives the uplinked FDM signal via an uplinkreceive antenna 114 for a feeder-link. The uplink signal is amplified112 and sent through a FDM receiver 110 that de-multiplexes the closelyspaced carriers and frequency converts them to the satellite segmentdownlink frequency. The de-multiplexed and frequency-converted signals,identified by “A” as “pre-WF demuxer signals,” are divided into twopaths. The signals in the first path 150 are sent to an onboard N′-to-N′WF muxer. In the second path, the de-multiplexed and frequency-convertedsignals are then passed through a wavefront de-multiplexer 108, whichperforms a spatial FT of the received signals. The outputs of thewavefront de-multiplexer 108 are amplified 106 and routed to elements ofa broadcast antenna array 104. The BWVs that were encoded into the twobeams by the ground segment facility 116 operate to generate twoindependent downlink beams 130 and 132 in the user segment 134. Thefootprints of the two downlink beams 130 and 132 can be controlled bythe ground segment 116 to illuminate geographically separated users 136and 138. The ground segment 116 can also control which digital contentis sent to user groups 136 and 138 by varying the inputs 140 and 142 tothe two DBF processors 124 and 126.

It should be noted that while the embodiment described above employedtwo DBF processors to encode two uplink content streams, more than twoDBF processors and content streams may also be used to create more thantwo downlink beams. Such systems employing multiple DBF processors wouldalso fall within the scope and spirit of the present invention.

FIG. 5B depicts 3 processing blocks 510, 530 and 540; the block 510 ison board a satellite, the blocks 530 and 540 are on ground. A set of WFmux/demux operation features a D/L WF muxing 513 on board satellite inthe block 510, and the associated D/L WF demuxing 533 on ground in theblock 530.

For the on-board down-link unit 510, the inputs of de-multiplexed andfrequency-converted signals, identified by “A” as the onboard primedata, featuring M individual streams are sent to an onboard N′-to-N′ D/LWF muxer 513; where N′>M and N′ inputs also consisting of N′−M pilotsinputs initiated on satellite. The N′ outputs of the D/L WF muxer 513are frequency-converted and FDM muxed by an FDM muxer 514, amplified bya downlink transmitter 515, and then radiated by the feeder-link antenna114.

The captured signals by the ground antenna 128 are sent to a downlink(D/L) processor 530, in which the received signals are amplified by aD/L receiver 531, demuxed and frequency converted by a FDM demuxer 532into N′ signal streams which are frequency converted to a commoncarrier, and then connected to a programmable equalizer 533A as a pathdifferential compensating processor, and the D/L WF Demuxer 533. Theprogrammable mechanism for the equalizer 533A is implemented via adynamic downlink compensating weighting vector (D/L CWV). There is anoptimization loop, converting outputs associated with satelliteinitiated pilot signals from the D/LWF Demuxer 533 to cost functions asperformance indexes by a cost function generator 535, which are used todynamically form a Total D/L Cost as a good current performanceindicator of the optimization loop. Cost functions must be positivedefined (≧0). An optimization processor 534 based on a cost minimizationalgorithm calculates updated D/L CWV for the equalizer 533Acontinuously. At steady state, total D/L cost shall become zero ornegligibly small. As result, the updated D/L CWVs shall remain the same,and the M-streams of onboard pre-WF demuxer signals 550 are recovered onground.

A main optimization loop 540 features M-stream inputs from the recoveredonboard prime data 550, a WF demuxer 543 to emulate performance of onboard WF demuxer 108, and a cost function generator 545 convertingoutputs of the WF demuxer 543 associated with ground initiated pilotsignals from the WF muxer 122 to cost functions as performance indexes.Cost functions are used to dynamically form an uplink (U/L) total cost,and must be positive defined (≧0). An optimization processor 544 basedon a cost minimization algorithm continuously calculates updated CWV U/Lfor the programmable equalizer 543A. At steady state, the U/L total costshall become zero or negligibly small. As result, the updated U/L CWVsshall remain the same, and are periodically sent to the pre-distortionequalizer 522A in the main path for updating its U/L CWV.

FIG. 5C presents a more detailed block diagram of an embodiment of aground segment portion of a digital broadcast system in accordance withthe present invention. The two content data streams 140 and 142 aredepicted as inputs to a first DBF processor 202 and a second DBFprocessor 206. The digital content stream 140 is split into ten portionsthat are multiplied by components of a ten-dimensional first BWV 204.Each of the ten components of the first BWV 204 is a complex numbercontaining both an amplitude piece and a phase piece. The specificvalues of the BWV are chosen to create a particular beam footprint fromthe downlink array antenna 104 of the satellite segment, as fullydescribed with reference to FIG. 5. The content data stream 140 ismultiplied in the array of complex multipliers 214 by the first BWV 204,and the encoded contents are then routed to a wavefront multiplexer 216.

At the same time, the second input content stream 142 is multiplied by asecond set of BWVs 208 in the second DBF processor 206 to create anindependent second beam in the satellite segment. Note that altering thevalues of the second BWV 208 will change the footprint and pointingdirection of the second beam generated by the satellite segmentindependent from those of the first beam. After multiplication by thesecond BWV 208, the outputs of the second DBF 206 are combined withthose from the first DBF 202 on an element-by-element basis, and thecombined element signals are routed to the inputs of the wavefrontmultiplexer 216.

The BWV 1 204 and BWV 2 208 may also be used to include static and timevarying equalizations for amplitude and phase differentials due tounbalanced paths and electronics on board satellites between the outputsof the WF demuxer 108 and the radiating array elements 104.

The wavefront multiplexer 216 takes the input elements, encoded withamplitude and phase pointing data, and performs a spatial Fouriertransform (FT). The wavefront multiplexer 216 can be implemented in manyways, such as by a sixteen-by-sixteen muxers using Butler Matrices asbuilding blocks, well known in the art. Note that for the embodimentdepicted in FIG. 5C, only ten of the inputs of the wavefront multiplexer216 are used while all outputs are used. The remaining six inputs areset to null (zero) inputs, as shown schematically at 218. The zero-inputchannels, served as probing signals, will allow for continuouscalibration, equalization, and optimization of the dynamic propagationeffects of the uplink back-channels in the feeder link, enablingimproved system performance Wavefront multiplexers of sizes other thansixteen by sixteen may also be used, different numbers of unused inputsmay also be employed, and continuous calibration may be performed aslong as the number of wavefront multiplexer signal inputs is less thanthe number of outputs. Such alternately sized systems would also fallwithin the scope and spirit of the present invention.

By forming a Fourier transform, the wavefront multiplexer has the effectof mixing each of the ten inputs into each of the sixteen outputs whichare connected to an equalizer 522A with a bank of digital weightingcircuitries continuously performing pre-compensation weighting functionsby modifying the 16 outputs by a compensation weighting vector (CWVU/L), which are controlled by a real time optimization loop 540. Thus,each of the ten input element signals is replicated and goes through allof the sixteen output channels, and each of the zero-input channels alsogoes through all of the sixteen output channels. In the subsequentground processing, uplinking to the satellite element, and processing inthe satellite, portions of the null-input signals thus sample each ofthe sixteen uplink channels and can be used to correct for dynamicchannel-specific propagation effects and imbalances.

The outputs of the wavefront multiplexer 216 are run throughdigital-to-analog converters 216 that synthesize analog waveforms. Theanalog waveforms are then frequency up-converted 224 by a series ofclosely spaced carrier frequencies in the feeder-link uplink frequencyband and generated by frequency sources 220. The uplink frequency may beany frequency spectrum useful for satellite communications, such asS-band, C-band, X-band, Ku-band, or Ka-band. For purposes ofillustration, the uplink will be assumed to be C-band at 6 GHz. Thesixteen output channels are then frequency up-converted by sixteencarriers separated by 62.5 MHz and extending from 5.75 GHz to 6.25 GHz.Other channel spacing can be used depending on the bandwidthrequirements of the application. The separate carriers are then combinedinto two 500-MHz-wide C-band signals by low-loss frequency-domainmultiplexers (FDMs) 226. The FDM carrier is then amplified 228 by asolid-state power amplifier, traveling wave-tube amplifier (TWTA),klystron, or other radio-frequency amplifier known in the art, andtransmitted via an uplink transmit antenna 128. The two 500-MHz C-bandsignals are uplinked to the designated satellite using polarizationdiversity. For example, one signal may be horizontally polarized and theother vertically polarized. Or one may be right-hand circularlypolarized (RHCP) and the other left-hand circularly polarized (LHCP).

Note that the ground segment described above may operate by itself or inconjunction with multiple ground segments having similar configurations.As long as their uplinks are separated via multiple frequencies/codes ormultiple beams, multiple ground segments can be used to uplink contentto the same satellite or group of satellites.

The optimization loop 540 utilizes the recovered onboard prime data 550as real time inputs, performing diagnostics on amplitude and phasedifferentials mainly caused by the propagation paths in the up-linkportion of the feeder-link via a duplicated WF demuxer 543 emulating theperformance of the onboard demuxer 108. The optimization loop 540converts some outputs of the WF demuxer 543 into cost functions asperformance indexes, sums the all cost functions into a total cost, andthen performs measurements of cost gradients, which are used tocalculate the updated U/L CWV 543B continuously based on a costminimization algorithm by an optimization processor 544. At steadystate, U/L total cost shall become zero or negligibly small. As aresult, the updated U/L CWVs 543A and 543B shall remain the same, andare periodically sent to the equalizer 522A in the main path forupdating its U/L CWV.

FIG. 6 presents an implementation concept of onboard equalization onsatellite. It depicts more detailed block diagram of an embodiment of asatellite segment portion of a digital broadcast system shown in FIG. 3with onboard equalization in accordance with the present invention. Theuplink receive antenna 114 receives the C-band FDM uplink signal fromthe ground segment depicted in FIG. 2. Note that frequencies other thanC-band may be used for the uplink feeder links and still fall within thescope and spirit of the present invention. On the broadcastingsatellite, the two FDM uplinked signals at two orthogonal polarizationsare amplified by low-noise amplifiers 302 and sent to two FDMde-multiplexers 304 that split the 6 GHz FDM signals into constituentcarriers, spaced 62.5 MHz apart. Of course, different frequency spacingof the constituent pieces of the uplink carrier may be used within thescope of the present invention.

The sixteen individual components of the uplink carrier are then passedthrough sixteen frequency converters 306 that frequency-shift the uplinkelements to the broadcast downlink frequency, here assumed to be S-band.However, other satellite broadcast downlink frequencies such as Ku-bandor Ka-band may also be used.

The outputs of the frequency converters 306 are the sixteen channelsignals, all at S-band in this embodiment. The sixteen channels arepassed through band-pass filters 308 and then modulated by acompensation weighting vector (CWV U/L) via a programmable equalizers620. The equalized outputs then enter a wavefront de-multiplexer 312that performs a spatial Fourier transform (FT) of the sixteen uplinkedchannels. The transformation performed in the wavefront de-multiplexer312 is essentially the opposite of that performed in the wavefrontmultiplexer 216 in the ground segment. Thus, ten element signals arerecovered, are filtered by band-pass filters 318, are amplified bysolid-state power amplifiers 320 or other radio-frequency amplifiers,and are routed to corresponding radiating elements of the downlinkbroadcast antenna array 322. The amplitude and phase profiles impartedto the signals by the ground-based DBF processors then cause the fieldsradiated by the downlink broadcast antenna array 322 to designatedcoverage areas beam-by-beam. The satellite segment itself is relativelyuncomplicated and simply passes through the beam-forming encodinggenerated by the ground segment. This keeps most of the complexity andcontrol on the ground, where it is easily accessible for upgrades,maintenance, and reconfiguration as required.

As the sixteen uplink element signals pass through the wavefrontde-multiplexer 312, the six null channels 314, or zero signal channels,are also recovered along with the ten signal channels. If all of theprocessing and propagation channels for the sixteen radiated signalelements were identical, the null channels would output nothing.However, imbalances in attenuation levels, phase delays and otherpropagation effects of the various uplink back channels will tend tocause energy to leak into the null channels 314. Thus, they becomeobservables, which in-turn can be used to measure and correct forimbalances in the uplink back channels. The null channels 314 are routedto a cost-function processor 316 converting these observables intoperformance indexes or cost functions which must be positively defined.The cost function conversions 314 usually feature non-linear mappingprocedures. An on-board optimization loop 610 will sum-up all currentcost functions as a current total cost 613, performing measurements onthe associated cost gradients with respect to the current CWV, andcalculating a new CWV according to a cost minimization algorithm in anoptimization processor 614, and finally updating CWV 615 accordingly innext cycle.

As previously noted, a system in accordance with the present inventionneed not employ ten signal channels, sixteen uplink element signals, andsix null channels. In more general terms, a system may comprise N signalchannels and M uplink element signals, where M and N are positiveintegers greater than one. If the continuous calibration method is to beused, M must be greater than N, and the number of null channels is equalto M−N. However, systems for which M is equal to N and no continuouscalibration is performed would also fall within the scope and spirit ofthe present invention.

FIG. 7 presents a second implementation concept of onboard equalizationby circuitries on satellite. It depicts more detailed block diagram ofan embodiment of a satellite segment portion of a digital broadcastsystem shown in FIG. 3 with on-board equalization in accordance with thepresent invention. The uplink receive antenna 114 receives the C-bandFDM uplink signal from the ground segment depicted in FIG. 2. Note thatfrequencies other than C-band may be used for the uplink feeder linksand still fall within the scope and spirit of the present invention. Onthe broadcasting satellite, the two FDM uplinked signals are amplifiedby low-noise amplifiers 302 and sent to two FDM de-multiplexers 304 thatsplit the 6 GHz FDM signals into constituent carriers, spaced 62.5 MHzapart. Of course, different frequency spacing of the constituent piecesof the uplink carrier may be used within the scope of the presentinvention.

The sixteen individual components of the uplink carrier are then passedthrough sixteen frequency converters 306 that frequency-shift the uplinkelements to the broadcast downlink frequency, here assumed to be S-band.However, other satellite broadcast downlink frequencies such as Ku-bandor Ka-band may also be used.

The outputs of the frequency converters 306 are the sixteen channelsignals, all at S-band in this embodiment. The sixteen channels arepassed through band-pass filters 308 and then divided into two paths, amain path, and a diagnostic path 721. The 16 channel signals in the mainpath are modulated by a dynamically corrected uplink compensationweighting vector (U/L CWV) via an equalizers 620. The equalized outputsthen enter a wavefront de-multiplexer 312 that performs a spatialFourier transform (FT) of the sixteen uplinked channels. Thetransformation performed in the wavefront de-multiplexer 312 isessentially the opposite of that performed in the wavefront multiplexer216 in the ground segment. Thus, ten element signals are recovered, arefiltered by band-pass filters 318, are amplified by solid-state poweramplifiers 320 or other radio-frequency amplifiers, and are routed tocorresponding radiating elements of the downlink broadcast antenna array322. The amplitude and phase profiles imparted to the signals by theground-based DBF processors then cause the fields radiated by thedownlink broadcast antenna array 322 to designated coverage areasbeam-by-beam. The satellite segment itself is relatively uncomplicatedand simply passes through the beam-forming encoding generated by theground segment. This keeps most of the complexity and control on theground, where it is easily accessible for upgrades, maintenance, andreconfiguration as required.

The 16 channel band-passed signals in the second path 721 are sent to adiagnostic block 710. A set of the 16 channel signals are buffered in abank of FIFO 711 for a processing frame which is defined as the timeperiod for updating a new U/L CWV in the equalizer 620. A processingframe may consist of multiple cycles of optimization iterations, usuallybetween 10 and 200 cycles. The diagnostic blocks feature an equalizer712A in duplication of the equalizer 620 but with a dynamic U/L CWValtered iteration by iteration, and a U/L WF Demuxer 712 in duplicationof the one 312 in the main path. In each optimization iteration cycle,the buffered data in the FIFO buffers 711 are modulated by a U/L CWV inthe equalizer 712A, and then sent to the WF Demuxer U/L 712. The sixnull channels are also recovered along with the ten signal channels. Ifall of the processing and propagation channels for the sixteen radiatedsignal elements were identical, the null channels would output nothing.However, imbalances in attenuation levels, phase delays and otherpropagation effects of the various uplink back channels will tend tocause energy to leak into the null channels. Thus, they becomeobservables, which can be used to measure and correct for imbalances inthe uplink back channels. The null channels are routed to a costfunction processor that converting these observables into performanceindexes or cost functions which must be positively defined. The costfunction conversions usually feature non-linear mapping procedures. Anon-board optimization loop 710 will sum-up all current cost functions asa current total cost 713, perform measurements of the associated costgradients with respect to the current CWV, and calculate a new U/L CWVfor the equalizer 712B according to a cost minimization algorithm in theoptimization process 714, and finally updating U/L CWV in the equalizer712A accordingly. At the end of a processing frame, the new CWV U/L ofthe equalizer 712B will also be sent to update the CWV U/L of theequalizer 620 in the main path.

As noted previously, a system in accordance with the present inventionneed not employ ten signal channels, sixteen uplink element signals, andsix null channels. In more general terms, a system may comprise N signalchannels and M uplink element signals, where M and N are positiveintegers greater than one. If the continuous calibration method is to beused, M must be greater than N, and the number of null channels is equalto M−N. However, systems for which M is equal to N and no continuouscalibration is performed would also fall within the scope and spirit ofthe present invention.

FIG. 8 presents a block diagram of an embodiment of a digital broadcastsystem using two separated satellites in accordance with the presentinvention. There are three segments; a ground segment 810, a satellitesegment 820, and a user segment 830. In the ground segment 810, the twogroups of content data streams P1 140 and P2 142, and a group of pilotsignal streams P3 143 are depicted as inputs to a WF mux 1 814, whichare connected to a first DBF processor 8124 and a second DBF processor8126. The digital outputs from the first DBF processor 8124 are for afirst satellite 8210 and those from the second DBF processor 8126 arefor a second satellite 8220. The two satellites are located at differentorbital slots where all users over a service area in the user segmentcan access to both satellites concurrently via multiple-beam-antennas(MBAs).

Depicted are two such users 8310 and 8320 in the user segment. Areceived (Rx) only terminal for a first user 8320 features a dishantennas 8326 with two Rx beams, a multi channel D/L receiver 8325, anequalizer 8324A and an associated WF demux 1 8324. The outputs comprisetwo groups of recovered signal streams P1 and P2, as well as a group ofrecovered pilot signals P3. The measured differences between therecovered pilot signals and those of desired pilot signals which areknown a prior are mapped into performance indexes, which are used tomeasure and correct for imbalances among the propagation paths throughthe two broadcasting satellites. The performance indexes or the costfunctions must be positively defined. The conversions from observablesto cost functions usually feature non-linear mapping procedures. Anoptimization processor 8327 will sum-up all current cost functions as acurrent total cost 713, performing measurements of the associated costgradients with respect to current weighting parameters in the equalizer8324A, calculating a new weighting parameters including those foramplitude and phase (A & φ) modifications according to a costminimization algorithm, and finally sending the new parameters forupdating in the equalizer 8324A accordingly in next iteration cycle.

As a result of the WF multiplexing/demuxing with built-in pathdifferential equalization process, recovered signal streams P1 141 atthe outputs of the WF demux 1 8324 have passed through both broadcastingsatellites 8210 and 8220, and they are the results of coherent combiningof the radiations from both satellites. Similarly, the recovered signalstreams P2 142 at the outputs of the WF demux 1 8324 have passed throughboth broadcasting satellites 8210 and 8220, and they are the results ofcoherent combining of the radiations from both satellites. Recoveredsignal streams P1 141 and P2 142 at the outputs of the WF demux 18324share the power resources from both satellites.

The recovered signals P1 141 and P2 142 are sent to a set top box 8323which is connected to a media center. The media center delivers desiredsignals to various display devices 8322.

At the ground segment, the two DBF processors 8124 and 8126 shallfeature M1 outputs corresponding to M1 radiating elements 104 on bothsatellites 8210 and 8220. In our example M1=10. The numbers of inputs tothe two DBF processors 8124 and 8126 shall be number of independentbeams to be generated by the antenna arrays 104 on both satellites 8210and 8220. Each beam radiated by a satellite antenna array is associatedwith a beam weight vector (BWV) consisting of M1 complex componentsindicating amplitude and phase weighting for individual elements of thearray 104. The specific values of the BWV are chosen to create aparticular beam footprint from the downlink transmission antenna arrays104 of the satellite segment. For generating P1 beams from the array 104in the first satellite 8210, there are P1 BWVs; each with M1 complexcomponents. Similarly, for the second satellite 8220 with P2 beams fromthe array 104 with M1 elements, there shall be P2 BWVs and each BWV alsofeatures M1 complex components.

The outputs of DBF 1 8124, which are element signals for the array 104of the first satellite 8210, become part of the inputs to a first16-to-16 WF muxer 8111 The 16 channel outputs from the first WF muxer8111 are then frequency converted and FDM muxed by FDM muxers 8121,filtered and amplified by uplink transmitter 8131, and radiated by amultiple beam antenna (MBA) 8160 to a first designated satellite 8210.Similarly, the outputs of DBF 2 8126 are part of the inputs to a second16-to-16 WF muxer 8112. The 16 channel outputs from the second WF muxer8112 are then frequency converted and FDM muxed by FDM muxers 8122,filtered and amplified by uplink transmitter 8132, and radiated by theMBA 8160 to a second designated satellite 8220.

In the satellite segment, the two satellites of the present embodimentare located at different orbital positions but servicing same coverageareas concurrently. For the first satellite 8210, the received 16channel signals from the antenna 8201 of the feeder-link between thefirst satellite 8210 and a ground segment facility 810 are amplified andFDM de-multiplexed by an uplink receiver 8209. The demuxed 16 channels,converted to a common carrier frequency, are connected a dynamicequalizer 8208A for compensations of amplitude and phase differentialsamong 16 parallel paths incurred in the uplink between the uplinktransmitter 8131on the ground segment and the uplink receiver 8209 ofthe first satellite 8210. Ten of the 16 outputs of the WF demuxer 8208are the recovered element signals which are power-amplified by a bank ofpower amplifiers 106, and then radiated by the array elements 104. Theradiated signals are spatially combined in far-field over variousdesignated coverage areas for different beams.

As to the outputs of the WF demuxer 8208, the remaining 6 ports are notdepicted. They are for the recovery of pilot signal streams, which areused to derive performance index of current status of the dynamicequalizer 820A. Cost functions and associated total cost are generatedfor a quantified current status on the equalizer performance Costgradients with respect to the current compensation weight vector (CWV)of an onboard equalizer 8208A are then measured. An optimizationprocessor will update the CWV of the equalizer 8208A based on a costminimization algorithm.

Similarly for the second satellite 8220, the received 16 channel signalsfrom the antenna 8201 of the feeder-link between the second satellite8220 and the ground segment facility 810 are amplified and FDMde-multiplexed by an uplink receiver 8209. The demuxed 16 channels,converted to a common carrier frequency, are connected a dynamicequalizer 8208A for compensations of amplitude and phase differentialsamong 16 parallel paths incurred in the uplink between the uplinktransmitter 8132 on the ground segment and the uplink receiver 8209 ofthe second satellite 8220. Ten of the 16 outputs of the WF demuxer 8208are the recovered element signals which are power-amplified by a bank ofpower amplifiers 106, and then radiated by the array elements 104. Theradiated signals are spatially combined in far-field over variousdesignated coverage areas for different beams.

What is claimed is:
 1. A method for generating first and second beams,comprising: applying a first beam weight vector to first data so as togenerate second data for said first beam; applying a second beam weightvector to third data so as to generate fourth data for said second beam;performing first Fourier transform of first information associated withsaid second and fourth data so as to produce multiple first outputs; andperforming second Fourier transform of second information associatedwith said first outputs so as to produce multiple second outputs to berouted to an antenna array.
 2. The method of claim 1 further comprisingfrequency-up converting said first outputs, followed by said performingsaid second Fourier transform of said second information associated withsaid first outputs.
 3. The method of claim 2, after said frequency-upconverting said first outputs, further comprising combining said firstoutputs into a composite frequency-domain multiplexed signal, followedby said performing said second Fourier transform of said secondinformation associated with said first outputs.
 4. The method of claim2, after said frequency-up converting said first outputs, furthercomprising combining said first outputs into two streams for verticaland horizontal linear polarizations, followed by said performing saidsecond Fourier transform of said second information associated with saidfirst outputs.
 5. The method of claim 2, after said frequency-upconverting said first outputs, further comprising combining said firstoutputs into two streams for right-hand and left-hand circularpolarizations, followed by said performing said second Fourier transformof said second information associated with said first outputs.
 6. Themethod of claim 1, after said performing said first Fourier transform ofsaid first information associated with said second and fourth data,further comprising radiating third information associated with saidfirst outputs to a satellite, followed by said satellite performing saidsecond Fourier transform of said second information associated with saidthird information.
 7. The method of claim 6, after said radiating saidthird information associated with said first outputs to said satellite,said satellite demultiplexing fourth information associated with saidthird information, followed by said satellite performing said secondFourier transform of said second information associated with said fourthinformation.
 8. The method of claim 1, wherein said first beam weightvector comprises a complex number.
 9. The method of claim 1, whereinsaid first beam points in a different direction from said second beam.10. The method of claim 1 further comprising combining said second andfourth data, followed by said performing said first Fourier transform ofsaid first information associated with said second and fourth data. 11.The method of claim 1 further comprising said performing said firstFourier transform of said first information associated with said secondand fourth data and null data.
 12. A method for generating first andsecond beams, comprising: applying a first beam weight vector to firstdata so as to generate second data for said first beam; applying asecond beam weight vector to third data so as to generate fourth datafor said second beam; performing Fourier transform of first informationassociated with said second and fourth data so as to produce multipleoutputs; and radiating second information associated with said outputsto a satellite.
 13. The method of claim 12 further comprisingfrequency-up converting said outputs, followed by said radiating saidsecond information associated with said outputs to said satellite. 14.The method of claim 13, after said frequency-up converting said outputs,further comprising combining said outputs into a compositefrequency-domain multiplexed signal, followed by said radiating saidsecond information associated with said outputs to said satellite. 15.The method of claim 13, after said frequency-up converting said outputs,further comprising combining said outputs into two streams for verticaland horizontal linear polarizations, followed by said radiating saidsecond information associated with said outputs to said satellite. 16.The method of claim 13, after said frequency-up converting said outputs,further comprising combining said outputs into two streams forright-hand and left-hand circular polarizations, followed by saidradiating said second information associated with said outputs to saidsatellite.
 17. The method of claim 12, after said radiating said secondinformation associated with said outputs to said satellite, saidsatellite demultiplexing third information associated with said secondinformation.
 18. The method of claim 12, wherein said first beam weightvector comprises a complex number.
 19. The method of claim 12, whereinsaid first beam points in a different direction from said second beam.20. The method of claim 12 further comprising combining said second andfourth data, followed by said performing said first Fourier transform ofsaid first information associated with said second and fourth data.